I'm installing a softener loop off the 3/4" main in my front yard. There is a 1/2" pipe branching off the 3/4". A plumber said it was a separate line for the outside spigots AND the kitchen sink. Could this be right

Joel,Well, it could be right. The plumber at the home has a lot better chance of guessing right than I will. Ways to tell if the 1/2" line goes to the kitchen sink would be to:1) trace the line back.2) cap of the line and see what is affected.